Orthogonal frequency division multiplexing (OFDM) is a wideband modulation scheme in which a frequency bandwidth allocated for a communication session is divided into a plurality of narrow band frequency sub-bandwidths. Each narrow band frequency sub-bandwidth includes a radio frequency (RE) subcarrier. Subcarriers in different sub-channels are mathematically orthogonal to each other.
OFDM is a multi-carrier modulation scheme involving converting data to be transmitted into an M-ary quadrature amplitude modulation (QAM) complex symbol, converting a complex symbol sequence into a plurality of parallel complex symbols through serial-to-parallel conversion, and performing rectangular pulse shaping and subcarrier modulation of each parallel complex symbol. In the multi-carrier modulation, a frequency interval between subcarriers is set such that all of the subcarrier modulated parallel complex symbols are orthogonal to each other. Accordingly, OFDM allows the individual spectrums of subcarriers to overlap without inter-carrier-interference (ICI). This is due to the orthogonality of the subcarriers and also allows a high data transmission rate and high bandwidth use efficiency since a frequency bandwidth is divided into a plurality of orthogonal sub-bandwidths.
In data transmission systems (e.g., cyclic prefix (CP)-OFDM or time domain synchronous (TDS)-OFDM systems) using OFDM, a receiver identifies the characteristics of a channel by estimating a channel impulse response (CIR) while estimating the channel based on a known data signal transmitted from a transmitter. However, in addition to the transmitted information used to estimate the CIR, the received signal may also include noise such as adjacent channel interference or white Gaussian noise, which may make channel estimation difficult.
FIGS. 1A through 1C are graphs illustrating received OFDM signals including noise and CIR information. FIG. 1A is a graph illustrating the amplitude of a CIR in a TU6 channel model, which is disclosed in “COST 207 TD(86)51-REW3(WG1): Proposal on Channel Transfer Functions to be Used in GSM Tests Late 1986, September 1986”. Referring to FIG. 1A, six channel paths exist in an exemplary channel (channel model) in terms of continuous time. FIG. 1B illustrates CIRs existing within a time period, during which no pulse exists in the channel model illustrated in FIG. 1A. These CIRs are unwanted CIR components having a low-energy value and are generated due to energy leakage from a period in which pulses exist. FIG. 1C is a graph obtained by observing the CIRs illustrated in FIG. 1B in terms of time variation. It can be determine from the graph that CIRs exist within the period in which no pulse exist in the channel model illustrated in FIG. 1A.
The above-mentioned CIRs being unwanted CIR components having a low-energy value may make it difficult to detect a CIR based on a known transmission signal generated by an OFDM transmitter. For instance, even if the method of threshold detection disclosed in “Z. Yang, J. Wang, C. Pan, et al., “Channel Estimation of DMB-T,” in 2002 IEEE Conf. Commu., Circuits and Systems and West Sino Expositions, pp. 1069-1072” is used, CIR detection may be difficult due to an unwanted CIR component, having a low-energy value because of inter-symbol interference (ISI) having a high-energy value and energy leakage in a channel model.